Electronic color images, such as television images, are typically generated using three electromagnetic beams that each represent a different primary color. For example, a color-television screen typically includes an array of pixels that are each split into three phosphorescent regions: red (R), green (G), and blue (B). Three corresponding electron beams, one each for R, G, and B, are aligned such that they simultaneously strike the R, G, and B regions of the same pixels as the beams sweep across the screen. These beams cause the R, G, and B regions of a pixel to phosphoresce, and the human eye integrates the light generated by the phosphorescing R, G, and B regions of all the pixels to perceive a color image. By adjusting the respective intensities of the beams, the color television can generate a pixel having virtually any color. Alternatively, the R, G, and B beams can be light beams that the human eye perceives and integrates directly.
FIG. 1 is a diagram of an image generator 100 that scans a viewable color image onto a display area 102 of a retina or display screen using R, G, and B light beams 104, 106, and 108, which are aligned in a common horizontal plane. A scanning mirror 110, such as a microelectromechanical (MEM) mirror, sweeps the beams 104, 106, and 108 onto the area 102 to generate the image. Because the beams are horizontally aligned and separated by an angle θ, the contents of each beam is delayed relative to the other beams so that the beams form color pixels that are spatially aligned. For example, as the mirror 110 sweeps the beams from right to left, the B beam strikes a location P on the display area 102. As it strikes the location P, the B beam has the proper intensity for the blue component of the image pixel located at P. At some time later, the G beam strikes the location P. Therefore, the content of the G beam is delayed relative to the content of the B beam such that the G beam has the proper intensity for the green component of the pixel as it strikes the location P.
A problem with the image generator 100 is that its maximum image scan angle φ is 2θ less than the maximum image scan angle of a single-beam image generator (not shown). The maximum scan angle φ is the angle over which the mirror 110 can scan an image onto the display area 102. Specifically, the rightmost portion of the area 102 is defined by the rightmost position of the B beam, i.e., the position of the B beam when the mirror 110 is in its right most position. Likewise, the leftmost position of the area 102 is defined by the leftmost position of the R beam. When the B beam is in its rightmost position, and is thus at the rightmost edge of the area 102, the R beam is 2θ beyond the rightmost edge of the area 102. Likewise, when the R beam is in its leftmost position, and is thus at the leftmost edge of the area 102, the B beam is 2θ beyond the leftmost edge of the area 102. Consequently, 2θ of the sweep angle of the mirror 110 is wasted. That is, if the mirror 110 scanned only a single beam—the R beam for example—then φ would increase by 2θ . This 2θ reduction in the maximum scan angle φ may be significant in applications such as a virtual retinal display (VRD) where the maximum scan angle φ of the mirror is small to begin with.
To overcome the problem of a reduced scan angle in a multi-beam image generator such as the generator 100, one can combine the multiple beams into a single, composite beam.
FIG. 2 is a side view of a conventional beam combiner 200, often called an X-cube, which combines the R, G, and B light beams 104, 106, and 108 into a single composite beam 202. For clarity, the center rays of the R, G, and B beams are shown in solid line, and outer rays are shown in dash line. For purposes of illustration, the outer rays are presumed to be substantially parallel to the respective center rays.
The X-cube 200 is a combination of four right-angle prisms 204, 206, 208, and 210 having vertices that meet at the center axis 212 (in the Z dimension) of the X-cube and form two interfaces 214 (dash line) and 216 (solid line). Before the X-cube 200 is assembled, the internal prism faces that form the first interface 214 are treated with an optical coating that reflects red light but passes green and blue light. Similarly, the prism faces that form the second interface 216 are treated with an optical coating that that reflects blue light but passes green and red light. Furthermore, either before or after the X-cube 200 is assembled, the external faces 218, 220, 222, and 224 of the prisms 204, 206, 208, and 210 are polished to an optical finish since they respectively receive and project the R, G, B, and composite beams of light.
First, the operation of the X-cube 200 is discussed where the R, G, and B beams 104, 106, and 108 include only their single center rays (solid line). This discussion also applies to thin beams—such as beams that are a single pixel wide—that are much narrower than the faces 218, 220, 222, and 224 of the X-cube 200. That is, this discussion also applies to collimated beams that are neither converging toward a focus nor diverging as they pass through the X-cube 200. The R, G, and B beams 104, 106, and 108 enter the X-cube 200 at the respective faces 220, 218, and 222, and the X-cube projects the composite beam 202 from the face 222. Specifically, the G beam 106 propagates through the face 218 to the center axis 212, passes through the interfaces 214 and 216, and exits the face 222 as part of the composite beam 202. The R beam 104 propagates through the face 220 to the center axis 212, and is reflected out of the face 222 by the interface 214 as part of the composite beam 202. Similarly, the B beam 108 propagates through the face 224 to the center axis 212, and is reflected out of the face 222 by the interface 216 as part of the composite beam 202. As long as the prisms 204, 206, 208, and 210 are properly dimensioned and aligned, the composite beam 202 is a single ray, i.e., is no wider than the R, G, and B beams 104, 106, and 108.
Therefore, referring to FIG. 1, one can use the X-cube 200 to increase the maximum scan angle of the image generator 100. Specifically, one can use the X-cube 200 to combine single-pixel R, G, and B beams 104, 106, and 108 into a composite beam that the scanning mirror 110 can sweep across an angle of φ+2θ as discussed above in conjunction with FIG. 1.
Next, the operation of the X-cube 200 is discussed where the R, G, and B beams 104, 106, and 108 are wider than a single ray (dashed line), i.e., have diameters/widths that are on the order of the widths of the faces 218, 220, 222, and 224. For example, such wide R, G, and B beams may respectively include the R, G, and B components of an entire image as opposed to merely a single pixel of the image. The operation is similar to that described above for the narrow-beam case, but because the R, G, and B beams are wider, they intersect the interfaces 214 and 216 at regions that are centered about the center axis 212. Furthermore, the interfaces 214 and 216 reverse the R and B beams such that the R and B image components in the composite beam 202 are the “mirror images” of the R and B image components in the R and B beams. But this reversal can easily be accounted for by “reversing” the contents of the R and B beams before they enter the X-cube 200.
Image-projection devices, such as overhead projectors, often include an X-cube that operates in the wide-beam mode.
Still referring to FIG. 2, a problem with the X-cube 200 is that the internal faces of each prism 204, 206, 208, and 210 typically must be precision machined and assembled to a high degree of flatness and angle accuracy to allow proper interfacing of the prisms. For example, the center vertex of each prism must be substantially a right angle (90°), and the internal prism faces must be polished to be substantially optically flat so that there are no gaps between the interfaces 214 and 216. Furthermore, because each prism has two internal faces that respectively form portions of the two interfaces 214 and 216, each prism must be twice treated with the respective optical coatings that produce the interfaces. In addition, each prism may be treated a third time with an anti-reflective coating on the respective external faces 218, 220, 222, and 224. Moreover, the prisms must be precisely aligned during assembly to insure even interfaces 214 and 216. Unfortunately, the precision machining, multiple treatments, and precision assembly typically make the X-cube 200 relatively complex and expensive to manufacture.
Another problem is that for the X-cube 200 to function correctly, it may be necessary to rotate the polarization of the G beam 106 by 90° (relative to the polarization of the R and B beams) before it enters the X-cube. One way to accomplish this rotation is to insert a half-wave retarder (not shown) into the path of the G beam before it enters the face 218. Unfortunately, this may increase the cost of an image generator that includes the X-cube.